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Turbulent Transport in a Strongly Stratified Forced Shear Layer with Thermal Diffusion

Abstract : This work presents numerical results on the transport of heat and chemical species by shear-induced turbulence in strongly stratified, thermally diffusive environments. The shear instabilities driven in this regime are sometimes called “secular” shear instabilities, and can take place when the Richardson number of the flow is large, provided the Péclet number is small. We have identified a set of simple criteria to determine whether these instabilities can take place or not. Generally speaking, we find that they may be relevant whenever the thermal diffusivity of the fluid is very large (typically larger than 1014 cm2 s-1), which is the case in the outer layers of high-mass stars (M ≥ 10 M), for instance. Using a simple model setup in which the shear is forced by a spatially sinusoidal, constant-amplitude body-force, we have identified several regimes ranging from effectively unstratified to very strongly stratified, each with its own set of dynamical properties. Unless the system is in one of the two extreme regimes (effectively unstratified or completely stable), however, we find that (1) only about 10% of the input power is used toward heat transport, while the remaining 90% is viscously dissipated; (2) that the effective compositional mixing coefficient is well-approximated by the model of Zahn, with D ≃ 0.02κT/J where κT is the thermal diffusivity and J is the Richardson number. These results need to be confirmed, however, with simulations in different model setups and at higher effective Reynolds number.
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Submitted on : Monday, May 23, 2022 - 10:28:35 AM
Last modification on : Wednesday, June 1, 2022 - 4:19:39 AM

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Pascale Garaud, Logithan Kulenthirarajah. Turbulent Transport in a Strongly Stratified Forced Shear Layer with Thermal Diffusion. The Astrophysical Journal, 2016, 821, ⟨10.3847/0004-637X/821/1/49⟩. ⟨insu-03675422⟩

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